Fast-neutron nuclear reactors cooled by a liquid metal such as sodium comprise a core consisting of fuel assemblies of a prismatic and elongated shape which are immersed in liquid sodium contained in the vessel of the reactor.
It may be necessary to remove fuel assemblies from the core of the reactor, inside the vessel, for example to replace old or faulty assemblies with new assemblies. The assemblies which have been taken out of the vessel of the reactor are in an irradiated state and produce radioactive radiation. Furthermore, these assemblies have residual activity and are the source of a release of heat produced by tho nuclear reactions in the irradiated fuel. Lastly, these assemblies retain, when they are taken out of the vessel, a certain quantity of liquid sodium.
The fuel assemblies taken out of the vessel of the nuclear reactor must therefore be arranged in containers ensuring the biological protection of the environment in which the assemblies are moved or stored temporarily. The assembly must also be cooled during its transfer.
For the transfer and temporary storage of fuel assemblies of fast-neutron nuclear reactors, casks are conventionally- used which comprise a solid body made of a material absorbing nuclear radiation and in which is formed a housing for a fuel assembly which may be bare or sunk in a casing filled with sodium in order to cool ,it.
The housing for the assembly, which is in the form of a central channel inside the body of the cask, opens out, via an aperture which can be plugged in sealing fashion, at one of the ends of the body of the cask, so as to permit the introduction of the assembly into the cask and its extraction. These operations are carried out after the transfer cask has been put in a position where it bears on a fixed structure, for example a structure integral with the slab closing the vessel of the reactor, in such a way that the central channel of the cask is situated in the axial extension of a through well of the fixed structure, for example of a vertical well traversing the slab.
A sealing device comprising at least one O-ring seal is inserted between the lower bearing surface of the support of the valve with which the transfer cask is equipped and the corresponding bearing surface of the fixed structure. The device for the sealed plugging of the aperture of the central channel of the cask opening out on the bearing surface of the support of the valve, and a second device for the sealed closing of the upper end of the vertical well of the fixed structure opening out on the bearing surface of the fixed structure, are arranged inside the zone of contact of the inner seal of the sealing device, in aligned axial positions.
In order to carry out an operation of removing or introducing a fuel assembly, the devices for plugging the central channel of the cask and the vertical well must be placed in their open position, after the transfer cask has been fitted onto the fixed structure. The plugging devices known in the prior art consist of valves, for example with a gate, a ball or a rotary plug, the plugging discs of which are separated by design or must be situated at a certain vertical distance from each other, after the cask has been fitted, in order to permit their operation both for opening and for closing.
Such an arrangement results in a rather large free space defined by the inner seal of the sealing device and by the outer surfaces of the discs when the cask has been fitted into position on the fixed structure; this represents a disadvantage insofar as all contact must be avoided between an oxidizing gas and the reactive liquid metal such as sodium in which the fuel assemblies of the reactor are immersed.
After transfer of the fuel assembly between the cask and the vessel of the reactor, the discs of the valves for plugging the cask and the passage well are closed again, with the result that a certain volume of gases which are polluted by radioactive substances is trapped in the space between the outer surfaces of the discs of the valves.
The free space between the outer surfaces of the discs of the valves must therefore be scavenged, both after the cask has been fitted, in order to remove any trace of oxygen in this space, and before the cask is disconnected, in order to remove any trace of gas likely to contaminate the surrounding environment with radioactive products.
This scavenging must be carried out using an neutral gas such as argon, with the result that a reserve of argon and scavenging means must be made available, which means must be connected to the cask or to the vertical well of the fixed structure.
Furthermore, this operation of scavenging the space lying between the closing valves of the cask and of the vertical well increases the time required for handling the fuel assembly.
The loading and unloading of the core of a nuclear reactor require numerous fuel assembly transfer operations, so that it is extremely important to reduce as far as possible the time required to transfer an assembly, in order to limit the downtime of the reactor for reloading or unloading.